1. Field of the Invention
The invention relates to a magnet system of a miniaturized sound transducer operating according to the electrodynamic principle. The sound transducer comprises a housing of deep drawn sheet metal, a magnet system comprising a magnet, and a diaphragm provided with a coil having wires extending to a contact. Such transducers are, in particular, used in connection with small devices such as telephones, mobile phones, hands-free devices for telephones, headsets or the like. Such miniaturized electro-acoustic sound transducers are provided with a diaphragm and a moving coil fastened to the diaphragm, a diaphragm holder, a magnet system, and optionally a front cover and/or a rear cover. The invention also relates to a method for manufacturing such a transducer.
2. Description of the Related Art
Electro-acoustic sound transducers operating according to the dynamic principle have been known for almost 100 years and are used in connection with loudspeakers, headsets or in different acoustic applications, for example, as the speaker part of a telephone. They are simply referred to in the following as transducers.
The current construction of such a transducer includes essentially plastic materials, as described in the publications AT 211 150 B and AT 236 474 B, for example. Such solutions have enabled manufacture of a transducer where the diaphragm seat as well as the transducer housing and also the injection-molded enclosure of the magnet system and of the electrical contacts provide a compact unit. These solutions have contributed to the fact that a semi-automatic transducer production has become widely accepted in practice instead of the prior manual manufacture. This solution requires plastic material injection molding devices that are sometimes very complex and thus expensive. A fully automated manufacture of transducers has not been realized up to now because of the complex manipulation of the transducer components to be embedded in plastic material. Accordingly, the essentially technically very excellent solution of embedded components cannot be used in a cost-efficient way in mass production.
In principle, it is important in order to increase, on the one hand, the output level of the transducer as well as, on the other hand, to minimize the distortion of the reproduction, to make the strength of the magnetic field in the air gap of the transducer as large as possible and as linear as possible. The magnetic field strength in the air gap of the transducer is primarily dependent on the magnetic strength and the volume of the permanent magnets. The stronger and the larger the magnet, the more potential is provided by it. This potential however can be used only when the length of the magnet lines to the air gap and the material in which they extend do not weaken them too much. This is the second important factor in order to maximize the magnetic field strength. Generally, long and “thin” paths are always bad because they weaken the magnetic force lines while passing through the magnetic yoke. When the magnetic properties of the employed materials are known, the computation of the magnetic fields is very similar to the computation of electrical circuits.
The magnetic circuits for electro-acoustic transducers are designed such that, by using a relatively narrow annular air area (gap), a maximum of magnetic field strength is achieved. In the case of small transducers and “expensive space”, the size of the magnet is very limited. All the more, the configuration of the magnetic yoke is important. In general, it is important that the yoke is not made unnecessarily thin. Therefore, magnet pot thickness of under 2 mm is considered bad and a thickness of under 0.5 mm is hardly usable because it causes too much loss. The same holds true also for the pole piece.
The second important role of the pole piece is to enable in the air gap of the acoustic transducer a magnetic field strength that is as linear as possible. This means that (in the transverse direction relative to the axis of symmetry, i.e., radially) the magnetic field distribution should be constant. Therefore, the pole piece has usually a thickness that matches approximately the stroke of the diaphragm. However, depending on the type and the application of the transducer, the stroke is between 0.5 mm and 2 mm. The moving coils are usually by 300% longer than the pole flange thickness. This is necessary in order to enable that even for extreme amplitudes of the diaphragm the coil is still positioned with at least one-third of its length within the air gap.
In the case of transducers having a diameter of more than 10 centimeters, it is therefore conventional to manufacture the housing by deep drawing sheet metal of a suitable thickness, usually a thickness of approximately 2 mm. The invention relates to miniaturized transducers having a diameter of less than approximately 20 mm; this does not allow for use of such sheet metal thickness for deep drawing. The use of thin sheets however is not indicated because of the aforementioned problems so that despite the resulting problems the aforementioned plastic housings are still employed.